Add a "rand" expression to the intensity value of your light:

Example 1:

rand(5)

This returns a random floating point number between 0 and 5, for example, 3.539.


Example 2:

rand(-1,1)

Returns a random floating point number between -1 and 1, for example, 0.452.

Example 2 would be better for your situation, as you can define a MIN and MAX intensity value for your light

So if you don't want it to go below an intensity value of 1, but also go NO higher than say, 4, you would use this rand expression:

rand(1,4)


Also, a messier and slightly less controllable way to add a random value to something is to map a ramp to an attribute, kill all ramp colors except for one, then map a fractal texture to that color.

But personally I would use example 2 from above

Good luck!